ArmA is Armed Assault, a high spec FPS made by the guys over at Operation Flashpoint. Here's a link to the official website...
http://www.armedassault.com
It's a great game, but my PC struggles in gunfights even though it bitchslaps the recommended system requirements. It runs better on some GFX cards than others though, so if you buy it, I would recommend a 512mb Graphics card to play it.
It's a great game, you can edit and create your own missions from scratch using waypoints, triggers etc., giving the AI a really realistic feel.
There are some super mods as well, Vietnam, WW2, and I have just finished installing the BushWar Mod (1966-1989 war between Angola and SA) and it's good fun.
ABM